1. Field of the Invention
The present invention relates generally to ultrasonic thickness determination, and more particularly to a method and apparatus for nondestructively determining the thickness of individual layers in multilayer articles.
2. Description of the Related Art
The determination of thickness by propagation of ultrasonic waves through an article is well known in the art. Typically, a transducer is used to generate an ultrasonic pulse which propagates through the article and is reflected off an acoustic interface and received by a receiving transducer. By knowing the velocity of the pulse through the article and the time of flight of the pulse, the thickness can be easily determined.
In many engineering applications, it is important to be able to monitor the thickness of individual layers in multilayer articles. For example, in the automotive industry, body panels and bumpers are commonly made from multilayer sheets which are extruded and subsequently thermoformed into a final shape. The top and bottom cap layers may comprise a pure LEXAN polycarbonate, for example, which produces a fine cosmetic finish, while the middle layer may comprise LEXAN reinforced with glass fibers for strength. During the thermoforming process, the multilayer sheets are typically stretched in a non-uniform manner, with some local areas experiencing a large degree of strain. If a cap layer is initially too thin, it may be strained to such a degree in some areas during thermoforming that cosmetic defects result, such as blistering or delamination. The ability to monitor and control the thickness of individual layers during the extrusion and thermoforming processes can thus be very important in avoiding cosmetic defects which can occur if the layer thicknesses deviate beyond specified tolerances.
Systems are known for ultrasonically monitoring layer thicknesses in certain multilayer articles. For example, U.S. Pat. No. 5,038,615 to Trulson et al. discloses a method for measuring the thickness of individual paint layers on a substrate. The Trulson method involves propagating ultrasonic waves through the object, averaging several pulse echoes indicative of each layer interface, and comparing the averaged waveform to a stored reference waveform.
The Trulson method, however, has limited applicability. For example, the Trulson method is designed to operate with layers, such as paint layers, which have very distinct interfaces, so that the pulse echoes correspond well with the stored reference waveform. However, in applications in which the interfaces between layers are indistinct and variable, the resulting pulse echoes have an uncertain shape, phase, and amplitude. In such applications, the Trulson method can not reliably correlate the pulse echoes with a reference waveform. The Trulson method also does not take into account the effects of attenuation of high frequencies of the pulse bandwidth, which can introduce significant measurement errors.
It would be desirable, therefore, to have a method and apparatus capable of determining the thicknesses of individual layers in a multilayer article in which the interfaces between the layers are unpredictable and indistinct, and where the layers may significantly attenuate the high frequencies of the pulse bandwidth.